Sequential or Random Access usage:

The ARC/WARC/GZip readers can be used to read either all the records/entries in a file sequentially or select records/entries in random order.

Both scenarios are supported by the various factory and reader methods.

Compression:

The ARC/WARC factory and reader methods support both compressed and uncompressed files in varying combinations. Basically you can create a reader for compressed or uncompressed files, but the factory also includes an auto-detection mechanism which can determine the correct reader on a per file basis.

Writing compressed ARC/WARC files is also possible though the use of different methods in the writer factories.

GZip compression is only supported on ARC/WARC files where each record is compressed individually and concatenated into one file and not the case where the whole ARC/WARC file and all it's records are GZip'ed as a whole. The later mostly because this makes random access to individual record highly ineffective.